The present invention relates to a method of producing a water-base resin composition for the non-chromate rust-preventive coating of metallic substrates, particularly for galvanized steel panels.
As the metallic surface treating agent, a chromium-containing surface treating agent such as a chromate system or a phosphate-chromate system has heretofore been used broadly and still in use today. However, in view of the recent trend toward more stringent regulatory control for environmental protection, it is likely that the use of such coating systems will be restricted for fear of the toxicity, particularly carcinogenicity, of chromium. Therefore, development of a rust-preventing agent not containing chromium and yet as effective as the chromium-containing treating agent in imparting corrosion resistance has been awaited. As disclosed in Japanese Kokai Publication Hei-11-29724, the inventors of the present invention previously developed a nonchromate rust-preventing agent comprising a water-base resin and, as incorporated therein, a thiocarbonyl group-containing compound, a phosphate ion, and water-dispersible silica. Regrettably, however, this system was found to be deficient in storage stability and somewhat poor in corrosion resistance at thin coating thickness, although it provides corrosion resistance equivalent to that afforded by a chromium-containing treating agent.
Meanwhile, with regard to the use of silane coupling agents in surface treating agents, an acidic surface treating agent containing two dissimilar silane coupling agents is disclosed in Japanese Kokai Publication Hei-8-73775. However, this system is intended to improve finger-print resistance and coating adhesion and is quite deficient in corrosion resistance for use in applications where high corrosion resistance is required after application of a rust-preventing agent as in the present invention.
Moreover, Japanese Kokai Publication Hei-10-60315 discloses a steel structure surface treating agent containing a silane coupling agent having a certain functional group reactive with an aqueous emulsion but the corrosion resistance required here is only that of a degree satisfying comparatively mild test requirements such as those of a wet test and as far as corrosion resistance is concerned, the system is a far cry from a rust-preventing agent meeting the highly critical corrosion resistance requirements, such as those of a salt spray test at thin film thicknesses, as provided by the present invention. With the above state of the art by way of background, there has been a standing demand for development of a nonchromate rust-preventing agent expressing sufficient corrosion resistance at thin coating thickness.
The present invention has for its object to provide a method of producing a water-base resin composition for rust-preventive coating which is suited for metal-coated steel panels, particularly galvanized steel panels, and despite the absence of chromium therein, is capable of imparting high corrosion resistance to coated metals, with the additional advantage of improved storage stability, and to a water-base resin composition as obtained by the above production method.
The production technology according to the present invention is a method of producing a water-base resin composition for rust-preventive coating which comprises reacting a water-base resin composition containing, in each liter thereof, 10 to 500 g of water-dispersible silica and 0.02 to 20 g of a silane coupling agent and/or a hydrolytic condensation product thereof at a temperature of not less than 50xc2x0 C. and not over the boiling temperature thereof. The water-dispersible silica mentioned above is preferably one member or a mixture of two or more members selected from the group consisting of spherical silica, chainlike silica and aluminum-modified silica.
The water-base resin composition according to the present invention is a water-base resin composition for rust-preventive coating as obtainable by the above production method.
In the present invention, the water-base resin composition contains water, which serves as a solvent, and a water-base resin. The water-base resin includes not only water-soluble resins but also resins which are intrinsically insoluble in water but are provided as finely dispersed in water, for example as emulsions or suspensions. The resin usable as such water-base resin includes polyolefin resin, polyurethane resin, acrylic resin, polycarbonate resin, epoxy resin, polyester resin, alkyd resin, phenol resin, and other thermosetting resins, and these can be used as a suitable mixture. The more preferred are crosslinkable resins which have functional groups, such as either carboxyl or hydroxyl, or both. Particularly preferred is a polyolefin resin, a polyurethane resin or a system comprising those two kinds of resins.
In the water-base resin composition of the invention, the water-base resin accounts for 1 to 80 weight parts, in terms of nonvolatile matter, and water accounts for 99 to 20 weight parts, in each 100 weight parts of the water-base resin and water combined. If the proportion of resin solids is less than 1 weight part, the corrosion resistance and top coat adhesion will not be sufficient. On the other hand, if the proportion exceeds 80 weight parts, the composition will be liable to undergo the undesirable gelation.
In the production method of the invention, a silane coupling agent and/or a hydrolytic condensation product thereof is used as the silane compound. The hydrolytic condensation product of a silane coupling agent means an oligomer obtainable by hydrolytic polymerization of the silane coupling agent.
The silane coupling agent which can be used as above in the present invention is not particularly restricted but includes the following, among others: vinylmethoxysilane, vinyltrimethoxysilane, vinylethoxysilane, vinyltriethoxysilane, 3-aminopropyltriethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-mercaptopropyltrimethoxysilane, N- (1,3-dimethylbutylidene)-3-(triethoxysilyl)-1-propanamine, N,Nxe2x80x2-bis[3-(trimethoxysilyl)propyl]ethylenediamine, N-(xcex2-aminoethyl)-xcex3-aminopropylmethyldimethoxysilane, N-(xcex2-aminoethyl)-xcex3-aminopropyltrimethoxysilane, xcex3-aminopropyltrimethoxysilane, xcex3-aminopropyltriethoxysilane, xcex3-glycidoxypropyltrimethoxysilane, xcex3-glycidoxypropyltriethoxysilane, xcex3-glycidoxypropylmethyldimethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, xcex3-methacryloxypropyltrimethoxysilane, xcex3-methacryloxypropyltriethoxysilane, xcex3-mercaptopropyltrimethoxysilane, xcex3-mercaptopropyltriethoxysilane and N-[2-(vinylbenzylamino)ethyl]-3- aminopropyltrimethoxysilane.
The particularly preferred silane coupling agent includes vinylmethoxysilane, vinylethoxysilane, 3-aminopropyltriethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-mercaptopropyltrimethoxysilane, N-(1,3-dimethylbutylidene)-3-(triethoxysilyl)-1-propanamine and N,Nxe2x80x2-bis[3-(trimethoxysilyl)propyl]ethylenediamine. These silane coupling agents can be used each alone or in a suitable combination.
In the present invention, said silane compound is used in an amount of 0.02 to 20 g (silica solids: ca 0.04 to 40 wt. %), preferably 0.1 to 2.5 g (silica solids: ca 0.2 to 5 wt. %), in each liter of the water-base resin composition. If the amount of the silane compound is less than 0.02 g, the contribution to corrosion resistance and top coat adhesion will be insufficient. On the other hand, if the limit of 20 g is exceeded, the shelf-life of the composition will be sacrificed.
In the method of producing a water-base resin composition according to the invention, water-dispersible silica is further employed. The water-dispersible silica which can be used is not particularly restricted but is preferably a spherical silica, chainlike silica or aluminum-modified silica which is weakly basic and lean in sodium and other impurities. The spherical silica includes colloidal silicas such as xe2x80x9cSnowtex Nxe2x80x9d, xe2x80x9cSnowtex UPxe2x80x9d (both manufactured by Nissan Chemical) and fumed silicas such as xe2x80x9cAerosilxe2x80x9d (Japan Aerosil); the chainlike silica includes silica gels such as xe2x80x9cSnowtex PSxe2x80x9d (Nissan Chemical); and the aluminum-modified silica includes xe2x80x9cAdelite AT-20Axe2x80x9d (Asahi Denka), for instance.
In the method of producing a resin composition of the invention, said water-dispersible silica is added at the amount of 10 to 500 g per liter of the water-base resin composition. If the amount of water-dispersible silica is less than 10 g, its contribution to corrosion resistance will not be sufficient. Over 500 g, the enhancement of corrosion resistance will reach a saturation point and an economic disadvantage will result.
In the method of producing a resin composition according to the invention, a solvent can be used for improving the film-forming properties of the resin to thereby yield a more uniform, smooth film. The solvent mentioned just above is not particularly restricted only if it is selected from among the solvents in routine use in coatings, thus including solvents in the alcohol series, ketone series, ester series and ether series, for instance.
Production of the water-base resin composition for rust-preventive coating according to the present invention can be carried out typically in the following manner. A reactor is charged with said water-base resin, solvent and water-dispersible silica and the mixture is heated to a temperature not less than 50xc2x0 C., preferably not less than 60xc2x0 C., but not more than the boiling temperature of the resin composition while it is constantly stirred. Then, a predetermined amount of said silane coupling agent is added dropwise over 1 to 8 hours while the system is stirred at the same temperature. On completion of the reaction, the product is cooled and, where necessary, adjusted with water or a solvent to a predetermined solid content, whereby the water-base resin composition of the invention is obtained.
In the production method of the invention, if the above reaction temperature is below 50xc2x0 C., the reaction between the silane compound and the water-base resin and/or water-dispersible silica will not proceed far enough so that the effect of the invention may not be obtained. On the other hand, if the reaction temperature reaches or exceeds the boiling temperature of the composition, the evaporation of water will be undesirably too vigorous. The reaction time need is 1 to 8 hours, and a predetermined amount of said silane compound is added within this time period at a rate of 0.1 g/min to 10 g/min. After completion of dropwise addition, the reaction is further continued for about 2 hours. Usually the reaction conditions may be 3 to 5 hours at 80xc2x0 C. The water-base resin composition for rust-preventive coating of the present invention can be used, either as it is or in combination with one or more other kinds of rust-preventing agents as a rust-preventive coating agent for metals.
By using the water-base resin composition for rust-preventive coating of the invention as a nonchromate rust-preventing agent, galvanized steel panels can be provided with a high degree of corrosion resistance exceeding that afforded by the conventional chromate-containing rust-preventing agent. Furthermore, as demonstrated, this water-base resin composition for rust-preventive coating has a good shelf-life. Thus, it is supposed that as the water-base resin, water-dispersible silica and silane coupling agent are mixed together and heated in the method of the invention, the silane coupling agent reacts with the water-base resin and, further, with the silica to enhance the density of crosslinks, with the result that both corrosion resistance and top coat adhesion are improved. Furthermore, as the unreacted silane coupling agent is eliminated, the shelf-life of this water-base resin composition for rust-preventive coating is dramatically increased.
The following examples illustrate the present invention in further detail without defining the scope of the invention.